1. Field of the Invention
The invention relates to a method for manufacturing a lithium ion secondary battery. In particular, the invention relates to a method for manufacturing a lithium ion secondary battery by performing pattern coating of a binder on a collector for a positive electrode or a negative electrode and then layering mix particles including an active material thereupon.
2. Description of Related Art
The growing popularity of hybrid automobiles and electric automobiles has recently increased a need for lithium ion secondary batteries that are used as drive power sources therefor, and a demand has accordingly been created for increased production efficiency, reduced size, and increased capacity of such batteries. A lithium ion secondary battery is manufactured by winding a positive electrode sheet and a negative electrode sheet, with a separator being interposed therebetween, inserting the resultant electrode body into a case, pouring an electrolytic solution, and sealing the case. A coated electrode manufactured by kneading an active material and a binder in a solvent to obtain a slurry-like paste, applying the paste in a thin-film form on the surface of a collector such as an aluminum foil or a copper foil, and then performing drying and pressing is generally recognized as an electrode (positive electrode and negative electrode) of a lithium ion secondary battery. However, in the conventional coated electrode, heat convection from the solvent in the drying process tends to cause migration in which the active material and the binder become detached in the paste applied in a thin-film form and the binder segregates to the upper side of the film. Where the active material and the binder are detached, the active material can be easily peeled off from the separator following expansion-contraction of the active material during charging and discharging, thereby reducing the electrode service life. Further, where the binder segregates to the upper side of the film, the conduction paths by which lithium ions are transferred through the electrolytic solution are unlikely to be formed in the electrode. The resultant problem is that the conventional coated electrodes cannot fully meet the demand for reduced size and increased capacity of lithium ion secondary batteries. Accordingly, Japanese Patent Application Publication No. 2004-79370 (JP-2004-79370 A) discloses a technique (the so-called two-layer coating method) of applying a first layer mainly including a binder on a collector surface and then applying a second layer including an active material and a binder thereupon, in which the first layer demonstrates a bonding strength higher than that of the second layer (the so-called two-layer coating method).
However, the problem associated with the technique disclosed in JP-2004-79370 A is that separate drying processes are required for the first layer and second layer, whereby the drying equipment is increased in size and the production efficiency is reduced. Further, since a method is used by which the active material, the binder, and the like are kneaded in a solvent, the resultant paste is applied, and the solvent is then evaporated in the drying process, the evaporated solvent itself and the drying energy are wasted.